Transcript Chiral Compounds, Optical Activity

Are different compounds with the same molecular
formula
Isomers
• Not mirror images
• Alkenes
• Cyclic? see 
• Mirror images
• All stereocentres have
opposite configuration
• ≥ 2 stereocentres
• Only some have opposite configuration
Constitutional (STRUCTURAL ISOMER) isomers
 Are compounds have the same molecular formula, but their
atoms are bonded in a different orders
 Thus they have the SAME MOLECULAR FORMULA but
DIFFERENT STRUCTURAL FORMULA
 Character of structural isomer
 Same molecular formula
 Structural isomers have different physical properties
 Different arrangement of atoms
 Or different position of functional group
 Or containing different functional groups

Constitutional isomers
Examples of Constitutional Isomers
1-Butene
2-Butene
pentane
2-methylbutane
2,2-dimethylpropane
The C6H14 Isomers
hexane
CH3CH2CH2CH2CH2CH3
(CH3)2CHCH2CH2CH3
2-Methylpentane
(CH3CH2)2CHCH3
3-Methylpentane
(CH3)2CHCH(CH3)2
2,3-Dimethylbutane
(CH3)3CCH2CH3
2,2-Dimethylbutane
Stereoisomers are isomers that have the same molecular
and
structural
formula
but
differ
arrangements of atoms in space
Types of Stereoisomers are:
1.
Optical isomers (chiral molecules)
A- Enantiomeres
B- Diastereomeres
2. Cis/trans isomer ( achiral molecules)
II-Stereoisomers:
only
in
the
Geometric Isomerism (cis/trans)
If two compounds have the same molecular and structural formula,
but differ in the spatial arrangement of different atoms or groups about
a ring or about a double bond
Example of Ring Cis-Trans Isomers
Cis: groups on same side of ring
Trans: groups on opposite side
of ring
cis-1,2-dibromocyclohexane
trans-1,2-dibromocyclohexane
If two identical groups occur on the
same side of the double bond the
compound is cis
If they are on opposite sides the
compound is trans
Example of Alkene Cis-Trans Isomers
cis-1,2-dibromoethene
Br
H
C
H
trans-1,2-dibromoethene
Br
H
C
C
CH3
H
C
Br
• Chirality
is a property present in an object that cannot be
superimposed on its mirror image.
• Chiral objects or chiral molecules do not have a plane of symmetry,
i.e. they are asymmetric.
• Chirality is typically seen in molecules that have a chiral or
asymmetric carbon atom.
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Optical isomers
Enantiomers are optically active, non-superimposable
mirror image molecules
 A molecule that has a non-superimposable mirror image is
chiral.
 Objects (and molecules) that are superposable on their
mirror images are achiral

Asymmetric Carbon
Many chiral compounds contain an asymmetric
carbon atom –
 Chiral or asymmetric carbons have four
different atoms or four different group
attached to it.
 It is a tetrahedral carbon atom

Optical activity
1. Enantiomers are optically active; that is, they
rotate plane-polarized light.
2. One isomer of an enantiomeric pair rotates
polarized light to the left (counterclockwise).
The other isomer rotates polarized light to
the right (clockwise). The degree of rotation is
the same but in opposite directions.
3. Rotation of polarized light to the right is
indicated by a (+) dextrorotatory
4. Rotation of polarized light to the left is
indiated by a (-) levorotatory .
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Optical Isomers
polarizing filter
plane
polarized
light
optically active sample in solution
Polarimeter: a device for measuring
the extent of rotation of planepolarized light
Dextrorotatory (d) = right rotation
Levorotatory (l) = left rotation
rotated polarized
light

RACEMIC MIXTURE = equal amounts of two
enantiomers; These mixtures are
optically
inactive.

Because two enantiomers rotate plane-polarized
light to an equal extent but in opposite
directions, the rotations cancel, and no rotation is
observed.

RESOLUTION: Separation of a racemic
mixture into its optically active forms (pair of
enantiomers)

DIASTEREOMERS (non-enantiomeric stereoisomers )

1. Molecules have more than one stereogenic (chiral) center:

2. are stereoisomers that are not mirror images of each other.

3. Diastereomers must have opposite configurations at some
(one or more) stereogenic centers, but the same configurations
at other stereogenic centers
DIASTEREOMERS
Enantiomers & Diastereomers
For a molecule with 1 chiral center, 21 = 2
stereoisomers are possible
 For a molecule with 2 chiral centers, a
maximum of 22 = 4 stereoisomers are
possible
 For a molecule with n chiral centers, a
maximum of 2n stereoisomers are possible

Meso Compounds
•
A meso compound is a stereoisomer that is superimposable on
its mirror image due to an internal plane of symmetry (is an
imaginary plane that bisects a molecule in such a way that the
two halves of the molecule are mirror images of each other).
A meso compound is an optically inactive compound even
through it possesses more than one chiral centre.
COOH
H
OH
plane of symmetry
H
OH
COOH
meso-tartaric acid
COOH
COOH
COOH
H
C
OH
HO
C
H
H
C
OH
H
C
OH
HO
C
H
HO
C
H
COOH
COOH
A meso compound
(2R,3S)(pl ane of symmetry)
COOH
COOH
HO
C
H
H
C
OH
COOH
A pai r of enantiomers
(2R,3R)-
(2S,3S)-
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rotation ab out
single bonds
Compounds with the
same molecular formula
rotation ab ou
sin gle b on ds
Compoun ds w ith the
same molecu lar formula
same
connectivity
same
Conformations
different
connectivity
d ifferen t
conn ectivity
Con fo
rotation
restricted
conn ectivity
Constitu tional
Conformational
Isomers
Constitu tion al IsomersConfo
Stereoisomers
Stereoisomers
stereoisomers
tereoisomers
butsno
chiral centers
bu t no ch iral centers
Cis,Trans
Cis,Trans
(E,Z)
Isomers
(E,Z)
(can bIsomers
e called
(can b e called
diastereomers)
diastereomers)
Isomers
Iso
with chiral centers
w ith ch iral centers
Atropisomers
m ore than
mcenter
ore than one ch iral center
on e chiral
on e chiral cen ter
achiral
chiral
ach iral
Meso
Meso
Compounds
Comp ou nds
Atrop
one ch iral center
ch iral
EnantiomersEn a
mirror
not mirror
not mirror mirror
images images
imagesimages
Diastereomers
D iastereomers
En antiomers
En antiomers
CHO
CHO
H
C
OH HO
C
H
H
C
OH HO
C
H
CH2 OH
CH2 OH
A pai r of e nantiome rs
(Erythre ose )
CHO
CHO
H
C
OH HO
C
H
HO
C
H
C
OH
CH2 OH
H
CH2 OH
A pai r of e nantiome rs
(Thre ose )

1) All but one of the 20 amino acids that make up
proteins are chiral and left-handed .

2) The molecules of natural sugars are almost all
classified as right-handed (R configuration).

3) the active site of enzyme is chiral, and only one
enantiomer of a chiral reactant fits it

Thus
Enzymes are capable of distinguishing
between stereoisomers
The biological importance of chirality